A history of the development of wind energy is presented by the publication entitled, "Wind Energy Developments In The Twentieth Century", published by the Lewis Research Center of the National Aeronautics and Space Administration, Cleveland, Ohio, in 1979. A predecessor to the subject wind turbine system, termed the "Mod-2 Wind Turbine", is described both in that publication and in another publication simply entitled, "Mod-2", published by the Bonneville Power Administration, U.S. Department of Energy.
The "Mod-2" wind turbine system comprises a 300 ft. diameter turbine rotor composed of two variable pitch blades. The first Mod-2 generator was incorporated into the Bonneville Power Administration grid in early 1981. Since that time, two additional Mod-2 generators have been incorporated into the grid.
A fixed pitch wind turbine rotor offers a simplification and lower cost over that of a controllable blade pitch wind turbine rotor. However, a fixed pitch rotor is harder to start because the blade pitch for efficient operation is different from that for good starting. A proposed solution to this problem is to use the generator that is driven by a wind turbine rotor as a motor during startup to bring the wind turbine rotor up to operating speed. This concept is proposed in U.S. Pat. No. 4,228,391, granted Oct. 14, 1980, to Witney H. Owen, in conjunction with the use of an induction machine having two sets of windings.
A fixed pitch turbine rotor is designed to stall in high winds, to limit rotor torque. Therefore, the electrical generator and drive system have to be large and strong enough to take the maximum torque that the rotor can generate. Thus, the generator must not only act as a starter but must be capable of absorbing the maximum power output of the rotor. The rotor in turn must have the capability to not critically overspeed in the event the load is for some reason removed from it. One proposed way of limiting the rotational speed of a wind turbine rotor is to provide the rotor with a centrifugal force operated drag brake. This concept is disclosed by U.S. Pat. No. 2,058,500, granted Oct. 27, 1936, to Frederick C. Plucker, in conjunction with a small wind turbine system.
Various systems have been proposed for improving performance of wind-driven generators by correlating the power requirements of the generators to the power output characteristics of the wind-driven rotors. Examples of these systems are disclosed by U.S. Pat. No. 4,095,120, granted June 13, 1978, to Kevin E. Moran and Eugene C. Korzeniewski; U.S. Pat. No. 4,146,264, granted Mar. 27, 1979 to Eugene C. Korzeniewski, and by the several patents described in these two patents.
U.S. Pat. No. 2,148,804, granted Feb. 28, 1939, to Edward M. Claytor, discloses a wind-generator system in which the load on the generator is varied in accordance with wind velocity. Specifically, when the wind velocity is low, the loading on the generator is low. When the wind velocity is high, the load on the generator is increased. A wind velocity control switch is provided for changing the load on the generator. A single battery or group of cells is connected to the generator when the wind velocity is low. At a predetermined higher wind velocity, the wind operated switch controls a circuit which effects the connection of the generator with a plurality of batteries in series.
U.S. Pat. No. 2,178,679, granted Nov. 7, 1939, to the same Edward M. Claytor, discloses a wind-driven generator control system which switches shunt resistors in and out of the field circuit in accordance with wind speed, in order to produce a modified generator loading curve which approximates the focus of the peaks of the propeller curves.
U.S. Pat. No. 2,339,749, granted Jan. 25, 1944, to John R. Albers discloses a system in which field current is controlled through a resistance varied by means of a governor-operated cam so that, as a rotor speed increases, resistance is gradually increased, then decreased.
U.S. Pat. No. 4,095,120, granted June 13, 1978, to Kevin E. Moran and Eugene C. Korzeniewski, discloses an electronic apparatus for matching a generator load curve to wind-driven rotor. The apparatus includes a rotor speed-responsive tachometer adapted to effect stepwise control of field current in the generator.
U.S. Pat. No. 4,146,264, granted Mar. 27, 1979, to Eugene C. Korzeniewski, also discloses a system which includes a tachometer for sensing the rotational speed of the generator and providing a control signal, but in a different manner.
Additional patents which should be considered for the purpose of putting the present invention in proper perspective relative to the prior art are:
U.S. Pat. No. 1,746,991, granted Feb. 11, 1930, to Herbert E. Bucklen; U.S. Pat. No. 2,086,279, granted July 6, 1937, to Eugene F. McDonald, Jr.; U.S. Pat. No. 2,106,557, granted Jan. 25, 1938, to Palmer C. Putnam; U.S. Pat. No. 2,178,047, granted Oct. 31, 1939, to Elmer K. Malme; U.S. Pat. No. 2,484,197, granted Oct. 11, 1949 to Albert C. Veldhuis; U.S. Pat. No. 4,039,848, granted Aug. 2, 1977 to William R. Winderl; U.S. Pat. No. 4,059,771, granted Nov. 22, 1977 to Marcellus L. Jacobs and Paul R. Jacobs and French Pat. No. 2,288,877, deposited Oct. 23, 1974 by Pierre Patin.
An important aspect of the present invention is to utilize a multiple speed pole amplitude modulated induction machine as a generator in a wind turbine system. Machines of this type are well known, but they have been used as motors rather than as generators. The pole amplitude modulation (PAM) technique is well described in the following publications:
G. H. Rawcliffe, R. G. Burbidge, W. Fong, "Induction Motor Speed Changing by Pole Amplitude Modulation", Proceedings IEE (England) 1958; G. H. Rawcliffe, W. Fong, "Speed Changing Induction Motors; Further Development in Pole-Amplitude Modulation", Ibid, 1960; G. H. Rawcliffe, W. Fong, "Speed-Changing Motors: Reduction of Pole Number by Sinusoidal Pole-Amplitude Modulation", Ibid, 1961; G. H. Rawcliffe, W. Fong, "Close-Ratio S-Speed Single-Winding Induction Motors", Ibid, 1963; W. Fong, "Wide-Ratio 2-Speed Single-Winding Induction Motors", Ibid, 1965; Two-Speed Induction Motors Using Fractional-Slot Windings", Ibid, 1965; J. M. Shulman, J. W. Colwell, E. H. Marbel, "Two-Speed Motor Starting With Inertia Loads", IEEE A 78 611-6 Power Engineering Society; E. F. Merrill, R. L. Fuller, Jr., "Economics of Multi-Speed Motors for Power Plant Applications", Proceedings of the American Power Conference, Vol. 33, 1971; C. E. Pastor, R. E. G. Ratcliffe, "Applications for PAM Motors", Power Engineering, June, 1969.